Judas - as Oscar Wilde remarked when characterising disciples and those they claim to venerate - is the one who writes the biography. Sometimes, having pocketed the 30 pieces of silver, the rancorous former acolyte fails to deliver. In 1989 Iris Murdoch asked AN Wilson to be her biographer; in circumstances that remain obscure, despite his repeated retelling of the story in this new memoir, he began work but then desisted.

Iris's secretiveness, he claims, sabotaged the commission. She brazenly lied to deflect his attention from her abandoned lovers, and kept him from meeting her Dublin relatives by pretending that they did not exist. Now she is safely dead, Wilson feels free to blab out unseemly truths and indulge in lurid speculations that he would earlier have suppressed.

Relying on his own fictional intuition, he outs Iris as a lesbian sadist and as a Russian spy who, while working at the Treasury, supplied her Communist masters with information which she posted in a hollow tree in Hyde Park. Could the fourth man have been, as Wilson crudely calls Iris, a 'rather dykey' woman? The tale of her supposed treason sounds absurdly implausible, and Wilson has no evidence for it. He got the story, he blithely says, from 'one of her friends'. He writes as a self-professed friend of hers, which perhaps is why it does not occur to him that to transmit such vile and tawdry tattle is not an act of friendship.

When Wilson first knew Iris, he was intent on becoming a priest. She encouraged his sense of vocation, though in the end - as he reveals after obliging us to yawn through his dark night of the soul - he thought better of it. For a while, he mistook Iris for the deity's representative on earth: he once said, with a giggly whoop, that his relationship to her resembled that between Maria and the Mother Superior in The Sound of Music. Determined to climb every mountain, he now climbs all over Iris.

While gossiping about her promiscuity, her conniving cruelty, her lapping up of the cheapest booze and her gamey odour, he finds time to defame her husband, John Bayley, who was Wilson's tutor at Oxford, his generous mentor and - once upon a time, I assume - also his friend. How Judas would have rejoiced over such a deft dual betrayal.

His complaints against Bayley begin with pettifogging academic malfeasances. Bayley's critical books contain no footnotes, and Wilson piously contends that: 'We all need rigour.' Is his own unattributed tittle-tattle meant to represent such probity? He discredits Bayley's work indirectly, quoting attacks by others which he pretends to deplore.

Everyone in Oxford, we are told, was outraged when Terry Eagleton described Bayley as a spokesman for 'ruling-class orthodoxy' who was 'educated at Eton and became an officer in the Grenadier Guards'; this does not prevent Wilson - whose notion of rigour seems not to cover such matters as consistency - from later declaring that Bayley should not be mistaken for an unworldly trifler, because he is after all 'an Etonian and a former Guards officer'.

Likewise he quotes a journalistic piece by John Sutherland, slyly mocking the sexual athleticism to which the elderly Bayley laid claim in the third of his marital memoirs, Widower's House. Again Wilson deplores the 'donnish malice' of the source, but goes on to make related charges of his own which are undonnish and frankly malevolent. He implies that, during the last two years of Iris's life, Bayley was conducting an affair with the woman he later married. Inconsistent as ever, Wilson regales us with stories of his own adultery and divorce, but that's a different matter: he suffers, weeps, has nervous breakdowns, goes mad for a while, and so must be forgiven.

But how seriously can Wilson take his own tut-tutting about this infidelity, considering that elsewhere he preposterously suggests that Bayley is gay? The evidence, assembled with Wilson's customary rigour, is Bayley's jokey admiration for some trimly uniformed Israeli soldiers and a moment of tipsy 'enchantment' with a black waiter at an Oxford college.

In Peter J Conradi's biography, Iris's demon lover was Elias Canetti, whom she served as disciple and sexual slave. For Wilson, Bayley inherits the role of demonic abuser. Again the case seems trumped-up: Wilson belabours Bayley for thrusting Iris into depression and creative apathy by moving her from decaying country manor to poky suburban house where her study overlooked a children's playground. Iris, however, did not see this as his black-hearted revenge, and I remember her telling me it was John, loyally deferential as ever, who had done the renouncing. 'He doesn't have a study in the new house,' she said. 'But he doesn't mind.' She beamed at the thought of his generosity.

Wilson, reverting to the 'Why oh why?' mode of his tabloid columns, reports that he was 'sickened' when Bayley's memoirs made public the intimate details of Iris's decline. He is especially censorious when Bayley describes Iris's 'toilet habits', emphasising the infantile pride she took in exhibiting her stools. If I may adopt Wilson's emetic metaphor, I am nauseated by his account of the Bayleys' insanitary housekeeping on an evening when Wilson and his second wife 'supped' with them.

Yes, this is the kind of book in which people sup, though you have to wonder why such a prissy pair accepted an invitation to what Wilson calls a 'pigsty', where they were served a cube of pork pie, whiskery salami, and a Mr Kipling cake. Wilson, having surveyed the mud-black grease in the kitchen, checks out the bathroom, and reports with relish on the shit-encrusted loo.

The Bayleys' grime at least belonged to them. The ordure in Wilson's book, spicily mixed with bile, is all his own, and he has dumped it on two people whose only mistake was their kindness to him.